Remove SetupCmd.setup() and just use .run()

There is no need for the thin wrapper only removing the `args.` prefix
from variables. Just do everything in the .run() method.
Simplify setting the config in SetupCmd
Fix #51: Allow configuring GitHub API base URL

The setup command learned a new option `--baseurl` to configure
hub.baseurl, that's now used be the RequestManager to do the GitHub API
queries.

With this you can use an arbitrary base URL to use git-hub with any
crazy proxy configuration or GitHub Enterprise.